Because, even as a DM it takes me forever to remember the rules, 4-5 months is just when I'm settling in and getting started. So, I tend to play systems for at least a year or two.
The first RPG campaign I played in lasted 5 years in real life and covered 50 years in game, with multiple "time skips" lasting anywhere from 1 to 10 years.
The next campaign I was in was ran with a friend and I sharing gm duties and lasted 10 years in real life but only covered 3 years in game.
In both campaigns we were playing sessions about every 2-4 weeks for 3-5 hours at a time.
My newest campaign that I'm running was started over 6 years ago but we took an 18 month break because of a move to a new house and the time needed to pack, move, unpack and then set up an appropriate gaming space. It has covered about 1 year in game and, aside from the break, we play mostly weekly sessions of 2-3 hours each.
Since I came to the hobby through wargaming, I run my campaigns a bit differently in that I allow my players to use multiple characters so each player has 3-4 characters they run like a small squad and I can break the larger group into multiple smaller groups and each player can have a character in each small group and take part in every scene so everyone stays involved at all times.
A good system combined with a good game master , engaging story and motivated players can last for many years.
What I recommend to game masters is that they start with a good idea for a story with a beginning and desired end result and then let the players, characters and sessions fill in the details between, even altering the end result with their choices and actions, while the gm focuses on making each session build on the larger story even while focusing on small stories that involve the pcs.
To give an example, my current campaign is a Gamma World campaign I run for my family. Gamma World is a postapocalyptic sci-fi game and mine is set 500 years after a series of nuclear wars destroyed most of civilization. I have 5 players, each using 3-4 characters. The setting and story actually takes place during one of those "time skips" from my very first rpg campaign. I'm using the original group my friends and I made in 8th grade and used into our freshman year of college as npc guides and scouts for the new group. The story I'm using is that a zombie outbreak has occurred and the new group is now looking for a way to cure or end it while also trying to unite the valley into the kingdom we created in that original campaign.
It has gone extremely well and everyone looks forward to playing our family game night. Even the player that refers to it as "that stupid game" (as a joke) and did not initially want to play but did so to share in the family time, now enjoys the game and especially enjoys the combat encounters.
Tell an interesting story and use each session to give the pcs chances to do cool things with their abilities and affect the world around them and you have the makings of a long running campaign.
Or you can do nothing but "one shot" sessions where players make new characters regularly and you try different games and genres. Or you can do something in between those two extremes. It really depends on the group.
As a gm, I'm better when I can get to a point where I know the rules, characters and setting so well that I can improv an entire session with nothing more than a short paragraph of prep material. The more often I change rulesets, campaigns, and settings, the more time I have to spend planning the sessions and reading rulebooks. But, sticking with one setting and campaign becomes easier for me as the prep time drops the longer we play in that campaign.